This invention relates in general to electrostatography, and more specifically, to an electrophotoconductive imaging member having an improved charge generating layer.
In the art of xerography, a xerographic plate comprising a photoconductive insulating layer is imaged by first uniformly depositing an electrostatic charge on the imaging surface of the xerographic plate and then exposing the plate to a pattern of activating electromagnetic radiation such as light which selectively dissipates the charge in the illuminated areas. This electrostatic latent image may be developed to form a visible image by depositing finely divided electroscopic marking particles on the imaging surface.
A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer used in electrophotography is illustrated in U.S. Pat. No. 4,265,990. A photosensitive member is described in this patent having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are positioned on an electrically conductive layer with the photoconductive layer sandwiched between a contiguous charge transport layer and the conductive layer, the outer surface of the charge transport layer is normally charged with a uniform electrostatic charge and the conductive layer is utilized as an electrode. In flexible electrophotographic imaging members, the electrode is normally a thin conductive coating supported on a thermoplastic web. The conductive layer may also function as an electrode when the charge transport layer is sandwiched between the conductive layer and a photoconductive layer which is capable of photogenerating electrons and injecting the photogenerated electrons into the charge transport layer. The charge transport layer in this embodiment must be capable of supporting the injection of photogenerated electrons from the photoconductive layer and transporting the electrons through the charge transport layer.
Various combinations of materials for charge generating layers and charge transport layers have been investigated. For example, the photosensitive member described in U.S. Pat. No. 4,265,990 utilizes a charge generating layer in contiguous contact with a charge transport layer comprising a polycarbonate resin and one or more of certain aromatic amine compounds. Various generating layers comprising photoconductive layers exhibiting the capability of photogeneration of holes and injection of the holes into a charge transport layer have also been investigated. Typical photoconductive materials utilized in the generating layer include amorphous selenium, trigonal selenium, and selenium alloys such as selenium-tellurium, selenium-tellurium-arsenic, selenium-arsenic, and mixtures thereof. The charge generating layer may comprise a homogeneous photoconductive material or particulate photoconductive material dispersed in a binder. Additional examples of binder materials such as poly(hydroxyether) resins are taught in U.S. Pat. No. 4,439,507. Photosensitive members having at least two electrically operative layers as disclosed above in, for example, U.S. Pat. No. 4,265,990, provide excellent images when charged with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles.
Photoconductive particles dispersed in a film forming binder matrix are commonly employed in charge generating layers of photosensitive members having at least two electrically operative layers. A charge generating layer containing photoconductive particles such as trigonal selenium particles dispersed in a binder matrix such as polyvinylcarbazole exhibits excellent photoresponse properties. In the form of belts having welded seams, such photoreceptors perform very well in conventional copiers and duplicators which utilize relatively large diameter photoreceptor belt drive and support rollers and gentle cleaning systems. Although trigonal selenium particles have excellent sensitivity and good response to visible light, the particles are especially difficult to disperse in polymer binders. More specifically, when photoconductive trigonal selenium particles are dispersed a film forming binder, difficulties can be encountered in achieving a uniform dispersion of the particles in the binder matrix. Nonuniform dispersion of photoconductive particles in the binder adversely affects the uniformity of electrical response over different regions of the photoreceptor.
Other photoconductive particles for charge generating layers are phthalocyanines. Phthalocyanines exhibit good response to infrared light, but, like trigonal selenium particles, are difficult to disperse in polymer binders. It would be desirable to provide a charge generating layer which has a good response to both visible and infrared light. However, it is difficult homogeneously to disperse particles having different spectral responses in a binder because of their different compatibilities with binders. Typically, a binder that will suitably disperse one type of particle fails to adequately disperse a different type of photoconductive particle which is sensitive in a different spectral region. Separate charge generating layers have been proposed comprising photoconductive particles having good response in the visible region dispersed in a binder as one generating layer, and photoconductive particles having good response in the infrared region dispersed in a binder as another generating layer.
For flexible photoreceptors, many film forming binders such as polyvinylcarbazole tend to be brittle and can crack or delaminate during extended cycling or even during cutting of photoreceptor sheets from a photoreceptor web. Small diameter drive or support rollers (e.g. 19 mm or smaller) for belt type photoreceptors are desirable for effective automatic paper strip ping. However, photoreceptor belts formed by welding overlapping opposite ends of a cut photoreceptor sheet tend to prematurely fail at the seam when cycled around these small diameter drive or support rollers, particularly in embodiments where the charge generating layer does not adhere well to adjacent layers. For example, photoreceptors containing a charge generating layer comprising trigonal selenium particles dispersed in polyvinylcarbazole usually utilize an adhesive layer to improve the adhesion of the charge generating layer to the underlying charge blocking layer or conductive layer.
The characteristics of electrostatographic imaging members comprising photoconductive particles dispersed in a binder exhibit many deficiencies in automatic, cyclic electrostatic copiers, duplicators, and printers, particularly with respect to mechanical properties.
U.S. Pat. No. 4,786,570 to Yu et al, issued Nov. 22, 1988, discloses a flexible photoreceptor containing certain specific blocking and adhesive layer materials. Generator layers containing a trigonal selenium photoconductive material and binders such as polycarbonate, polyvinylchloride, vinylchloride and vinylacetate copolymer, styrene butadiene copolymer and the like are disclosed.
U.S. Pat. No. 3,816,118 to Byrne, issued June 11, 1974, discloses an electrophotographic plate containing phthalocyanine pigment dispersed in a binder. Numerous binders are disclosed, including polyvinylchloride, polyvinylacetate, poly-styrene-polybutadiene copolymer, and phenoxy resins.
U.S. Pat. No. 4,340,658 to Inoue et al, issued July 20, 1982, discloses a laminated photosensitive material for electrophotography wherein, for example, zinc oxide is dispersed in a resin binder. Binders applicable for the photosensitive material include copolymers of vinylchloride/vinyl-acetate, polycarbonates, or styrene/butadiene copolymers. The binders may be used singly or in the form of a mixture of two or more.
U.S. Pat. No. 4,701,396 to Hung et al, issued Oct. 20, 1987, discloses a photoconductive element containing dispersion coated flourine-substituted titanylphthalocyanine. Various binders such as styrene butadiene copolymer and polycarbonate are disclosed.